Legendary is the most difficult setting of Campaign gameplay in every Halo game to date, aside from the informal "Mythic" difficulty (a weekly challenge over Xbox LIVE) which refers to Legendary with all Skulls active in Halo games that feature them. The Legendary difficulty is also featured in Firefight and Skirmish games. At the end of a Halo game, there may be a cutscene that will only be available when a player has completed the campaign on Legendary.

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In the original Halo trilogy and Halo: Reach, the difficulty is symbolized by an Elite skull with a bullet-hole through one side, two crossed UNSCmachetes, and a shield bearing the Marathon symbol. In Halo 3: ODST it is symbolized by a Human skull with two crossed suppressed SMGs with the same shield. In Halo 4, it is represented as a shield with crossed blades and a Promethean skull. In Halo: The Master Chief Collection, the difficulty is represented by an elite skull with crossed Energy Swords over a Hunter's Shield.

It is regarded as a large challenge even for veteran players. Allies are practically useless aside from creating distractions. Overwhelming enemy firepower and superior numbers are plentiful on Legendary, while AI enemies have greater intelligence.
One must play cautiously and conservatively to succeed. Enemies are extremely strong while you and your allies' strength is extremely low in comparison, often leading the player to die frequently. Legendary on Halo 2 can be described as "Trial and Error." Completing the full Campaign on Legendary in all the Halo games unlocks an additional ending scene to that game's story. Halo: Reach and Halo 2 are the only exceptions to this.

In the first Halo game, the differences between Legendary and other settings is relatively numeric. The player does not face unique or different enemies but they do encounter many more enemies who are tougher to defeat and endure. Many of the same strategies that work on other settings generally function on Legendary in this game except that the player has to be more careful of one's own weaknesses than before. This is where the Plasma Pistol/Pistol combo really comes in handy. This game is arguably the hardest of the series since health packs are relatively scarce and shield regeneration is fairly slow, so the player must proceed with extreme caution, since medikits tend to be spread out.

Player shields are half their Normal resistance. A few hits from Plasma will drain them completely.

Enemy Shield resistance is doubled.

Extremely weak health. One melee from an enemy in front of you will often prove fatal.

Player shields and enemy shields recharge faster - twice as fast as when on Normal difficulty.

Enemy health is doubled.

Elites : Twice as many elites appear as on the Normal difficulty. Major Elites are encountered at a ratio of about one of every three elites encountered, improving AI. An area that had two Minor Elites on Normal should have four to five elites on Legendary with one or two of them being Majors. Their shields can take 28 rounds from an Assault rifle before lowering.

Grunts : Twice as many grunts appear than on the Normal setting. Around half of the Grunts encountered are Major Grunts.

Firing Rate : All enemies fire their weapons much faster and more accurately. In particular, Jackals will fire long streams of 10-15 shots with high accuracy. Jackals will also charge their plasma pistols much more often proving deadly in combat.

Agility : All enemies move much faster and will rarely allow a player to stick them with grenades. Elites will often do their dodging maneuver immediately if hit, making it very difficult to make consistent hits on them to drain their shields.

Stealth Elites : They are always encountered in packs of four to six with few exceptions.

Flood : The Flood are now armed 80 percent of the time. Their deadly melee attacks will drain shields by half. Many more of them wield Shotguns and Rocket launchers than before. Only a couple of Infection Forms can completely drain your shields, and an explosion from a Carrier Form can kill you instantly if your shields are completely drained.

Allies :Marines are virtually useless. They will constantly get stuck with grenades and most are easily killed by the enemy's increased rate of fire. Their returning fire only inflicts minimal damage on enemies. Their AI is increased significantly, still to little effect.

Zealots appear more frequently throughout the game. Incredibly, it takes 21 shots from a Pistol to remove their shields alone.

The player does not go through the controls tutorial section at the beginning of the level, the game assuming that anyone choosing the highest difficulty level has gone through it before. Instead, they will wake up and leave the cryo section immediately. This also occurs on Heroic difficulty.

Johnson's speech changes in the beginning of the level. He uses more graphic language and explains to the Marines that the Covenant will end up in puddles of their own blood. He also says, "Am I right, Marines?" with a more confident tone of voice.

Upon entering the loading bay, 4-6 Zealots will ambush the player and the marines in addition to the standard waves of grunts and jackals which pass through the hallways. There will also be a couple of Hunters in the hangar area where you first come up the lift.

The shotgun is no longer available early on in the game. The only way to pick it up is to kill a Combat Form that was holding one or from one of the dead Marines from the third room with an energy bridge.

The player only has five minutes to get to the Longsword fighter after destroying the Pillar of Autumn's engine core as opposed to the usual six minutes. The ending has also been drastically changed, now involving Johnson and an Elite.

In one of the Bungie Updates, Frankie describes Halo 2's Legendary as:

A whole new sick twist on game difficulty. Legendary includes bizarre stuff like perma-death for Co-Op players, meaning that you can't hopscotch like you can on other difficulty levels. Once a player dies on Legendary, both players are hurled back to the last checkpoint. It's brutal. Also, that place where you encountered two grunts and a flowerpot on Normal? Well now they're hunters, high-ranking, Sword carrying elites and they're all PMS-ing. Seriously, sticking your head around a corner on level two can get it shot clean off.

Ultra Elites are now a very common sight. You will encounter up to four in some parts of the game. They have the most powerful Shields in the game and can survive a Plasma Grenade and an astounding four headshots from a Sniper Rifle or Beam Rifle. They will also unleash their Energy Swords more frequently if they spot you. Also, Ultras and Zealots, most of the time, can survive 1 hit from a Sword, so you should think twice before approaching one of them. They can never survive, if hit in the back.

Elite Zealots now appear as enemies when playing as John-117. They always wield Swords and can slaughter large groups of Marines with ease.

Enemies are extremely adaptive and intelligent in tactical action. They rarely fall victim to being outsmarted.

Grunts retreat less often when a leader is killed.

Allies fire their weapons in much longer and more accurate bursts, providing players with excellent supporting firepower.

Marine allies are much stronger. They have more health and their AI is increased significantly. They are still highly vulnerable though. They also seem to take cover very often, and are also very adaptive, and careful when engaging enemies.

Sangheili allies, now fire more accurately, and throw more grenades.

Unggoy Allies, now when equipped with Plasma Rifles and Needlers, fire them much faster and accurately, like their enemy counterparts, when playing as John-117. They also throw more grenades than before.

Direct melee attacks received from Elites and Flood are always one hit fatalities.

Both players in co-op have "Linked Lives." That is, when one of the players dies, both are sent back to the last Checkpoint regardless of whether or not only one of them died (Possibly to prevent players from easily reaching the status of beating Legendary on their profile or to enhance it with the general knowledge of the above statement).

One shot from a Brute Shot will have the same power as a shot of the rocket launcher on Easy difficulty.

Enemies are several times more resistant to damage. (E.g. You need three body shots or two headshots for a fully shielded Brute Captain to take it down instead of one headshot or two body shots with a sniper or beam rifle).

Headshots are the only means to get a quick kill on enemies like Jackals and Grunts, aside from assassinations. As they can now survive several body shots from other weapons.

Nearly all enemy forces are promoted by one rank with high ranking Grunts and Brutes seen more often. This can be changed with the effects of the Thunderstorm Skull, which promotes enemies even higher. In addition to this promotion, Brutes will not carry Deployable Covers.

Marine allies are severely weakened, however when given weapons they'll fire more rapidly with greater accuracy, similar to how the enemy acts in Normal mode.

Enemy numbers double or sometimes triple in the case of Flood encounters.

Enemy AI improves dramatically, enemy vehicles will be significantly harder to hijack and enemies will be nearly impossible to stick with grenades.

Sniper Jackals, one of the rarer enemies in Halo 3, will appear with greater frequency, though not nearly as much as in Halo 2.

Enemies fire much faster and toss grenades more often and with greater accuracy.

Enemies fire weapons very accurately and at an extremely high rate of fire.

Turrets will appear in places where there once were none, including Shades. Gunners will also fire more aggressively with a rate of fire not possible by the player.

Brutes will wield Brute Shots more often. They fire them faster, more accurately and they often prove deadly; a single Brute shot will kill the player. Brutes will also frequently go berserk in even a random occurrence; getting hit by the Pure Tank Form's swipe attack will kill as well.

The "linked lives" from Halo 2 are removed but can be achieved with the Iron Skull activated.

Almost every Sentinel has shields, including allied ones.

In singleplayer mode, the Arbiter is next to useless and will frequently be knocked out for more prolonged periods. He will also take cover a lot more, which reduces his outgoing fire. However, this will also help protect him from getting knocked out as much.

In Halo Reach, enemy AI will be scaled according to the number of players (i.e. the more players there are, the more intelligent the enemy AI is), therefore it will not necessarily be easier to complete a mission on Legendary with more players.

Shade turrets fire significantly faster than physically possible by the player, and with greater sight range and accuracy. They will also notice you much sooner.

There is an achievement for completing the campaign alone on Legendary difficulty called "A Monument To All Your Sins," which is a reference to the Gravemind.

In a Bungie Weekly Update, it was stated that enemies on Legendary difficulty deal 77% more damage compared to normal difficulty. This is most likely yet another subtle seven reference.

Elite Minors are extremely rare on Legendary difficulty; the player will probably only see 5 or 10 of them throughout possibly the entire campaign (not counting Wraith and Banshee pilots). In their place are larger numbers of higher ranking Elites with more powerful defensive and offensive capabilities.

In Halo 3, you unlock EOD armor for beating the campaign on Legendary difficulty.

In Halo 3: ODST, you unlock Dare as a playable character in Firefight for beating the campaign on Legendary difficulty.

In Halo 2, Legendary is the only difficulty where you can find the hidden skulls, except for the Blind Skull and the Assassins Skull. The Assassins Skull does not have any effect when not playing on Legendary.

A similar style was also employed for Halo 3 DLC map packs, albeit with the difference of Battle Rifles, instead of SMGs.

Although the Halo: Reach difficulty logos follow the same pattern as those of Halo 3, the icon for the achievement, "A Monument To All Your Sins" (complete the campaign solo on Legendary), has a human skull instead of an Elite skull atop the shield and two combat knives.

The Legendary of Halo 2 is comparable to the Mythic challenge of later games.

Some rare dialogue can be heard during cutscenes and sometimes gameplay on Legendary difficulty.

Speed Running can be used to complete this difficulty in a less time consuming manner.

Halo 2 is considered to be the hardest Halo game to beat on Legendary difficulty.